I. Field of the Invention
The current invention relates to wireless telecommunications. More particularly, the present invention relates to a novel method and apparatus for real-time detection and location identification of in-band jammers in a wireless base station.
II. Description of the Related Art
Since the introduction of cellular phones to the marketplace, there has been an explosive increase in the usage of portable phones. The frequency spectrum available for wireless phone use, however, did not increase as quickly as the subscriber base. Eventually, the number of subscribers to wireless phone service began to outstrip the capacity of wireless infrastructure using the Advanced Mobile Phone System (AMPS) technology. In response to this imbalance, pioneering companies like Qualcomm developed ways of providing greater call capacity than possible with AMPS without requiring additional frequency spectrum.
In some cases, such as with Code Division Multiple Access (CDMA), this increase in efficiency was accompanied by an increase in complexity of the wireless equipment. By building powerful ASIC and microprocessor technology into both handset and base station equipment, such advanced wireless systems can utilize more powerful digital signal processing and communication system techniques to achieve better signal quality and capacity. Code division multiple access communications systems have been standardized in the United States in Telecommunications Industry Association TIA/EIA/IS95-A, entitled "MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM", hereafter referred to as IS-95 and incorporated by reference herein.
Some problems occur in any wireless communication system which can reduce the call-carrying capacity of the spectrum. One such problem is interfering, or jamming, transmissions introduced into the spectrum reserved for use by the wireless system. Transmitters not associated with the wireless system may cause such jamming transmissions by either purposely or inadvertently transmitting an unauthorized signal into the spectrum reserved for the wireless system. While the interference caused by the thermal noise inherent in any wireless system cannot be avoided, jamming signals may be removed, and the resultant lost capacity reclaimed, by turning off the jamming transmitter. Of course, urging the owner of a jamming transmitter to cease transmissions can only occur after the jamming signal is detected. Sometimes documented evidence of jamming is also required.
In a large, complex wireless system, however, it is not always easy to detect jammers. The jamming may occur sporadically, and be difficult to track. Currently, wireless base stations do not typically have the built-in ability to perform spectrum analysis of their received signals. The currently prevalent method of detecting jammers requires that the presence of a jammer first be suspected, by analyzing call capacity and dropped call logs of the system. When a base station is experiencing poor call quality or inexplicably high dropped call rates, a field technician may bring external spectrum analysis equipment to the suspect base station and connect it to the receive antenna system. Such spectrum analyzers are not built into every base station largely due to the amount of increased cost that such a design would require of base stations in an already cost-competitive market. The external spectrum analyzers used for jammer detection generally do not have connections to processors within the wireless system network, so the spectrum analysis data must be collected and analyzed off-line and manually by the field technician. Jamming signals that occur only sporadically may be difficult to detect using such methods, because they must be present when spectrum measurements are being taken to be detected. In addition, even if such a jamming signal is detected using such methods, the field technician gets little information about the location of the jammer from data collected at a single base station.
Most wireless receivers decode signals which occupy a predetermined frequency band. For this reason, the received signal is typically passed through a bandpass filter, which removes signals outside the spectrum reserved for the wireless system. Many receivers in such a wireless system are also equipped with automatic gain control (AGC) modules, which attenuate the incoming signal to better fit within the dynamic range of subsequent receiver circuitry. Such AGC modules attenuate the received signal so that it does not exceed the dynamic range of the subsequent sampling circuits, causing signal distortion commonly referred to as "clipping". In a preferred embodiment of the invention, an AGC formula is used which holds the root-mean-squared (RMS) value of the processed signal to a predetermined constant value. Jamming signals that reside within the wireless system's allocated spectrum, cannot be removed by bandpass filtering. Such in-band jammers will cause AGC circuits to attenuate the received signal more than would occur in the absence of jammers. The result is often a signal whose power spectral density has a shape that may be distinguished from that of a signal which is devoid of in-band jammer components.
In a system using spread-spectrum signals such as CDMA, all subscriber units transmit signals to the base station using the same frequency band and cause mutual interference to each other. In a CDMA system, call capacity is maximized by constraining all subscriber stations to transmitting the lowest power necessary to sustain a predetermined received SNR at the base station. An in-band jammer increases the noise level which every subscriber station must overcome to achieve this SNR, thus forcing every subscriber station to transmit at higher power levels.
An increase in subscriber transmit power causes several problems, including increased drain on batteries, which results in decreased standby and talk time of the subscriber stations. It also causes additional interference to subscribers operating in adjacent base station coverage areas. Subscribers operating in those adjacent base stations respond by increasing their transmit power m an escalating power race.
In addition, subscriber stations near the coverage boundary of a wireless base station may already be transmitting at their maximum level. If such subscriber stations cannot transmit enough power to maintain an acceptable SNR at the base station receiver, the link to that base station will drop. Thus, by increasing the level of transmit power needed to maintain an SNR level near the edge of coverage, a jammer may, in effect, cause shrinkage of a wireless base station's effective reverse-link coverage area.
In many CDMA systems, the forward link radius of the base station is deliberately decreased to match the shrinkage in the reverse link radius which occurs as the result of reverse link loading. Thus, a jamming signal could also result in shrinkage of the wireless base station's effective forward-link coverage area. The balancing of forward and reverse link cell radius is further disclosed U.S. Pat. No. 5,548,812, entitled "METHOD AND APPARATUS FOR BALANCING THE FORWARD LINK HANDOFF BOUNDARY TO THE REVERSE LINK HANDOFF BOUNDARY IN A CELLULAR COMMUNICATION SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.
As the number of transmitting subscriber stations increases, jammer detection becomes more difficult. This increased difficulty arises because the jammer represents a smaller percentage of the total received power, making it easier to hide. For this reason, a built-in spectrum analyzer, which can analyze the received spectrum during spontaneous lulls in call activity, is highly desirable.